1. Field of the Invention
The present invention relates to an engine exhaust apparatus for purifying exhaust gases, and more specifically, to an engine exhaust apparatus in which secondary air is introduced into the exhaust gas to accelerate purification of the exhaust gas by means of a catalyst or the like.
2. Information of the Related Art
Conventionally known is a technique in which secondary air is introduced into the exhaust gas delivered from an engine, whereby exhaust gas purification is accelerated in a post-treatment device, such as a catalyst or thermal reactor.
As a means for introducing the secondary air, there is a pump or some other auxiliary equipment in which the secondary air is positively fed into the exhaust gas. According to this arrangement, however, the flow of the secondary air must be controlled by means of a throttle valve or the like so that the secondary air intake corresponds to the exhaust gas delivery. This control operation is complicated, and the whole apparatus is inevitably large-sized, thus entailing high cost.
Another secondary air introducing means is designed so as to feed the secondary air by utilizing the ejector effect which is produced during the discharge of the exhaust gas. This is a simple structure which can introduce the secondary air at a rate corresponding to the exhaust gas delivery. Accordingly, this means is widely used in various engines.
An example of the means which utilizes the ejector effect is described in U.S. Pat. No. 3,468,124. In this case, an ejector section for introducing the secondary air by utilizing the ejector effect is provided on the upstream of an exhaust manifold. A muffler for lowering the exhaust noise level is arranged on the downstream of the ejector section.
Since the muffler with high flow resistance is arranged on the downstream of the ejector section, however, a satisfactory ejector effect cannot be obtained in introducing the secondary air.
Since the negative-pressure components of the exhaust pressure of the engine are minor, as shown in FIG. 9, moreover, the ejector effect is inevitably lowered by the positive-pressure components of the exhaust pressure.
Moreover, the body of the muffler is heated by the reaction heat between the exhaust gas and the secondary air with the aid of the catalyst, so that there are some problems on durability, including oxidative deterioration. Furthermore, the sounds of sudden expansion of the exhaust gas during its discharge are emanated through a secondary air intake passage to the outside, thus constituting exhaust noises.